1. Technical Field
The present invention relates to a printing apparatus and a method of controlling transport of print media for continuous printing.
2. Related Art
As an ink jet printer that performs printing on a print medium, such as a regular paper, there is a printer including an LD (load) roller serving to supply a print medium to the inside of the printer and a PF (paper feed) roller serving to transport the print medium supplied to the inside of the printer (for example, refer to JP-A-2003-72964 and JP-A-2006-117385).
In a printer disclosed in JP-A-2003-72964, an LD roller is connected to a PF motor, which drives a PF roller to rotate, through a clutch and may be disconnected from the PF motor. In the printer disclosed in JP-A-2003-72964, a print medium set on a paper feed tray is transported up to the PF roller by the LD roller connected to the PF motor. When the print medium is transported up to the PF roller, the LD roller and the PF motor are disconnected from each other. The printer disclosed in JP-A-2003-72964 transports the print medium, which has been transported up to the PF roller, using the PF roller.
In a printer disclosed in JP-A-2006-117385, an LD roller and a PF roller are rotatably driven by separate motors, respectively. That is, the LD roller is rotatably driven by an ASF (automatic sheet feeder) motor, and the PF roller is rotatably driven by a PF motor. Also in the printer disclosed in JP-A-2006-117385, a print medium is transported up to the PF roller by the LD roller and is then transported by the PF roller.
In a printer, it is requested to improve a throughput. The throughput is expressed as the maximum print number of sheets per unit time.
However, in the printer disclosed in JP-A-2003-72964, for every print medium, it is necessary to cause the LD roller to be connected to the PF motor through the clutch and perform a transporting operation using the PF roller in a state where the clutch is disconnected after rotating the LD roller. In addition, also in the printer disclosed in JP-A-2006-117385, for every print medium, it is necessary to transport a print medium using the LD roller and then perform a transporting operation using the PF roller. That is, a paper feed operation and an operation from printing to paper discharge after the paper feed operation are separate operations. Accordingly, when continuously printing a plurality of print media, the above operations should be repeatedly executed in the order. Due to a limitation on such control operation, there is a limit in improving the throughput in the printers disclosed in JP-A-2003-72964 and JP-A-2006-117385.
Accordingly, in order to overcome the limitation on such control operation, it is considered to keep driving the LD roller and the PF roller at the same time such that a plurality of print media are continuously transported. However, in the case of driving the LD roller and the PF roller at the same time, there is a possibility that, for example, the following problem will occur.
If an actual transport amount by which the PF roller transports the print medium becomes greater than an actual transport amount by which the LD roller transports the print medium, a state of a print medium after being transported changes such that the print medium is pulled between the PF roller and the LD roller, as compared with a state of the print medium before being transported. Due to the change in the pulling direction, a so-called back tension that causes the print medium to be pulled from the PF roller side to the LD roller side acts on the print medium.
In addition, the posture of the print medium on which the back tension acts with respect to the transport direction changes to cause a slanting transport or the actual transport amount of the print medium at the time of completion of the transport control becomes smaller than a target transport amount, according to an action of the back tension. The transporting precision of the print medium is lowered. In addition, damage occurs in the print medium on which the back tension acts or a pulling sound of the print medium occurs.